Fixed-frequency clocks are standard in virtually any piece of electronic equipment that includes digital circuitry, such as personal computers, cell phones, and microwave ovens. However, fixed-frequency clocks can cause electro-magnetic interference (EMI) in electronic devices.
Spread Spectrum Clock Generation (SSCG) is one technique that may be used to reduce EMI. Prior to the inception of SSCG, the principal methods used to mitigate EMI were shielding and filtering with passive components. However, these conventional techniques have been less practical to use when electronic devices were more complex, mobile, and/or operated at higher clock frequencies. Shielding, in particular, becomes unpractical as clock speeds increase and form factors decrease for electronic devices. Increased shielding adds to the size, weight and overall cost of an electronic device. Additionally, filtering often needs to be tuned or manually adjusted on a per-device basis to meet EMI requirements, further increasing manufacturing costs.
On the other hand, SSCG provides system-wide coverage as well as flexibility. Further, it does not interfere with signal-to-signal timing relationships (set-up and hold times). Since SSCG allows for less shielding and passive filtering; it can reduce the size and cost of an electronic device.
SSCG diminishes the EMI emitted by a digital system by frequency modulating the system's master clock with a low-frequency signal. This modulation changes the frequency spectrum of the resultant clock from a single spike to a reduced amplitude carrier with sideband harmonics. This reduction in the carrier amplitude brings substantial benefits in EMI control. Often, most or all of the other clocks in the electronic device are slaved to the SSCG master clock, and as a result, significant EMI reduction can occur throughout the device.